A conventional pressure relief valve in its simplest form typically includes a valve seated by an adjustable compression spring against the force of the fluid pressure of the system it protects. More sophisticated designs include internal mechanisms which require adjustment when the system fluid is changed. However, when adjusted for one fluid, such valves tend to become unstable when called on to handle another. In addition, repeat accuracy of the set or blow-off point is difficult to achieve in large spring-loaded valves because of non-uniform heating of the large, set-pressure adjustment spring. To confront many of the problems associated with self-activated spring loaded pressure relief valves, various pressure operated relief valves were developed. Pressure operated relief valves include a pilot valve acting in association with a main valve. Generally, pilot operated relief valves allow for superior quality in control parameters such as leak tightness, flow characteristics and stability.
Snap acting (popping) flowing spring-loaded valves were some of the first pilots used in conjunction with main valves. These pilots were generally utilized with main valves having a linear piston within a cylinder. Thereafter, the concept of providing modulation or proportional behavior in the pilot to prevent excessive amounts of product discharge captivated the field of research and design. It was apparent, however, that these latter concept presented insurmountable exploitation problems due to flow through the pilot during main valve discharge and the resultant failure of the pilot caused by contact with the industrial fluids and associated impurities. In light of these failures in the pilots, it became apparent that non-flowing pilots were needed to avoid contamination by industrial fluids flowing through the pilots.
Various pilot operated relief valves using non-flowing pilots have been developed which utilize diaphragms as sensors, an axial displacement design, soft seated pilot valves and often external tubing to interconnect the pilot to the main valve. Such devices, although avoiding many of the problems associated with pressure operated relief valves which allowed for fluid flow in the pilot, present problems in areas such as stability, adaptability to new and old systems, field service, long and short term reliability, efficiency, ease in adjustment, etc. These problems can be said to be due, to some extent, to the relatively complicated designs associated with these pilot operated relief valves.